This invention relates to the data communication and storage arts and, more particularly, to a method and apparatus for sensing the beginning of a valid digital data message after a period during which invalid data has been received.
Digital data is often transmitted and received in bit serial fashion. Such bit serial communications may take place between terminals and a central data gathering device; during wireless or wire data transmission between, for example, communicating computer systems; and between storage devices and a controller when the storage means is basically serially oriented in nature. Rotating memories, such as magnetic discs and magnetic drums, are examples of serially oriented memory storage devices.
Bit serial transmission of data specifically involves transmission and reception of serial streams of coherent information constituting a message with indeterminate time periods existing between valid messages during which invalid messages may be transmitted and received. For example, memory storage on a rotating disc may include a plurality of messages spaced in "sectors" disposed circumferentially about the disc on various tracks radially spaced from one another. Space must be left between adjacent sectors on a given track in order to separate correspondingly adjacently stored messages. However, the spaces are not physically delineated on the disc surface such that magnetic signals, constituting invalid messages, typically are found in such spaces.
Bit serial data communication, particularly between magnetic discs and disc controllers, is often carried out using a bi-phase (double frequency) code. That is, each cell time normally includes a clock pulse followed by a period in which the presence or absence of a pulse signifies a data "1" or a data "0".
Messages which are transmitted and received serially in bi-phase form must therefore include a message-termination code (which advises the receiving apparatus that a message transmission has been completed) and, more importantly, a code by which the eminent reception of a valid message is called to the attention of receiving apparatus which has previously been rejecting all invalid messages after the reception of the last previous message-terminated indication. Information transmitted and received prior to the transmission of a valid message for the purpose of so alerting the receiving apparatus and for effecting synchronization when necesary is called a "preamble". It is obvious that the preamble detection means must be virtually infallible whereas it is also desirable, for both economic and reliability purposes, to utilize preamble detection apparatus that is simple.